In: Respiratory14 Jul 2014
Respiratory syncytial virus (RSV) is a major cause of acute bronchiolitis and pneumonia in young children. The pathogenesis of acute bronchiolitis has generally been considered to involve RSV infection of airway epithelial cells,2 but more recently, several groups have established that RSV proteins also localize to alveolar macrophages (AMs) in vivo during acute bronchiolitis. Using a guinea pig model of experimental RSV infection, we have also demonstrated viral protein within AMs during acute bronchiolitis. AMs are cells derived from blood monocytes that protect the lower respiratory tract against pathogens (including viruses) and inhaled particles and maintain sterile conditions in the lung. Flovent inhalers website Despite the role of AMs in host lung defense mechanisms, several studies have reported that RSV can cause productive infection of human AMs and blood monocytes in vitro. However, only a minority of these cells are permissive to RSV infection, even with exposures to large amounts of virus, and the reasons for this limited permissiveness of AMs to RSV infection are poorly understood. Moreover, the compartmentalization of RSV proteins within infected AMs has not been evaluated by electron microscopy, such that our knowledge of virus-cell interactions at the ultrastructural level is limited.
It is well established that the population of AMs is normally heterogeneous and that subpopulations of these cells can be separated by density gradient centrifugation, in which the degree of cellular maturation achieved in vivo is inversely related to buoyant density. Since cord blood monocytes have been shown to be significantly more permissive to RSV infection than AMs, we hypothesized that the permissiveness of AMs to acute RSV infection in vitro may be inversely related to their degree of maturation achieved in vivo, where comparatively immature AMs would show increased permissiveness to the virus. To investigate this hypothesis, we performed BAL on guinea pigs and prepared subpopulations of AMs by centrifugation on discontinuous gradients of metrizamide. The permissiveness of these AM subpopulations to acute RSV infection in vitro was studied by using immunocytochemistry to enumerate cells containing viral proteins and viral plaque assays were used to quantify amounts of intracellular virus. To explore the intracellular localization of RSV proteins in the AM subpopulations after experimental RSV infection, we performed immunogold electron microscopy on ultrathin sections of glutaraldehyde-fixed cells.
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